This invention relates to a liquid-crystal display device and a process for producing the same. More particularly, this invention relates to an improvement of an assembly of liquid-crystal display panel in which a TAB as one of the IC mounting flexible resin films is connected to a liquid-crystal display panel. The invention also relates to a process for producing such an improved liquid-crystal display device.
The structure of a prior art liquid-crystal display panel is shown in FIG. 4. The liquid-crystal display device comprises basically a liquid-crystal display panel 1 having a plurality of electrodes 2 arranged in the end portion, a TAB 4 that has a liquid-crystal drive IC 5 mounted thereon and which also has a plurality of output terminals 3 arranged in the end portion (as lead wires on TAB 4), and an input circuit board. The output terminals 3 are connected to the electrodes 2 on the liquid-crystal display panel 1 by means of an anisotropic conductive adhesive 6. The output terminals 3 for the TAB 4 are formed on the obverse side of the TAB film (FIG. 4 shows the case where the lead wires are on the side opposite to the IC mounting side; in other cases, the lead wires may be on the reverse side).
Terminals on the input side (not shown) of TAB 4 are usually soldered to the output terminals on a board that is called either a column input circuit board or a row input circuit board. This input circuit board receives, via connectors or terminals, external signals for driving the electrodes (so-called X or Y electrodes) and supplies the input terminals on the liquid-crystal drive IC 5 with signals for driving the electrodes. In response to those signals, IC 5 generates and outputs signals for driving the respective electrodes that are connected in correspondence to the individual output terminals.
The liquid-crystal display panel 1 has a "column" panel which is such that part of the transparent electrodes arranged in the vertical direction (in column direction) is exposed at an end and a "row" column panel which is such that part of the transparent electrodes arranged in the horizontal direction (in row direction) is exposed at an end. These two panels have the liquid crystal held therebetween and are superposed in such a way that the sides where the transparent electrodes are formed (e.g., their electrode forming sides) face each other. The panels may be formed of such substrates as a glass substrate and a plastic film substrate. Part of the electrodes exposed at an end may be coated with a metal. All of such substrates are denoted by the word "panel" in the following description and the appended claims. The substrates are usually provided with a light polarizing plate on the outer surface.
FIG. 5 is a sketch illustrating the cross-sectional structure of the area where TAB 4 is connected to the liquid-crystal display panel 1. Arranged on the underside of TAB 4 are output terminals 3a, 3b, . . . (in FIG. 4, these output terminals 3a, 3b, . . . are collectively referred to as output terminals 3). Arranged on the surface of the end portion of the liquid-crystal display panel 1 are electrodes 2a, 2b, . . . (in FIG. 4, these electrodes 2a, 2b, . . . are collectively referred to as electrodes 2). TAB 4 is connected to the liquid-crystal display panel 1 by first placing them in superposition with the anisotropic conductive adhesive 6 interposed and with output terminals 3a, 3b, . . . on the TAB 4 being positioned in registry with electrodes 2a, 2b, . . . on the liquid-crystal display panel 1, and thereafter compressing the joined area under heating. As a result of this thermal compression, the anisotropic conductive adhesive 6 will cure to have the TAB 4 joined to the liquid-crystal display panel 1.
The anisotropic conductive adhesive 6 contains conductive particles 7 at a given density and, upon thermal compression, the conductive particles 7 will contact both the output terminal 3a and the electrode 2a which are disposed facing each other, whereby said output terminal and said electrode will conduct. Other facing combinations of output terminals and electrodes (e.g. the output terminal 3b and the electrode 2b) will conduct in a similar way via the conductive particles 7. However, adjacent output terminals (e.g. output terminals 3a and 3b) or adjacent electrodes (e.g. electrodes 2a and 2b) will not conduct. The density of the conductive particles 7 to be incorporated in the anisotropic conductive adhesive 6 is determined in such a way as to satisfy the above-described conditions. In the actual device, the pitch between output terminals 3 (3a, 3b, . . . ) and between electrodes 2 (2a, 2b, . . . ) is as small as ca. 0.25 mm (width, 0.125 mm; gap, 0.125 mm). Hence, very precise positioning and joining operations are required to establish connection between each output terminal and the corresponding electrode.
The anisotropic conductive adhesive 6 is made of a thermosetting epoxy or a thermoplastic polyester. The thermosetting anisotropic conductive adhesive provides high bond strength and insures high reliability in connection. However, even if some defect is found in an operating test following the joining process, the once bonded TAB is difficult to be disconnected from the liquid-crystal display panel, thereby making it considerably difficult for an operator to replace the defective TAB.
On the other hand, the thermoplastic anisotropic conductive adhesive usually has a comparatively weak bond strength, so it has the advantage that even if some defect is found in an operating test following the joining process, the defective TAB can be readily replaced. However, the thermoplastic anisotropic conductive adhesive has the disadvantage of low reliability in connection.